It is sought more particularly here below in this document to describe problems existing in the field of seismic data acquisition for oil prospecting industry. The invention of course is not limited to this particular field of application but is of interest for any technique that has to cope with closely related or similar issues and problems.
The operations of acquiring seismic data on site conventionally use networks of sensors (here below designated as “hydrophones” with regard to the acquisition of data in a marine environment). The hydrophones are distributed along cables in order to form linear acoustic antennas hereafter referred to as “streamers” or “seismic streamers”.
As shown in FIG. 1, the network of seismic streamers 20a to 20e is towed by a seismic vessel 21. The hydrophones are referenced 16 in FIG. 2, which illustrates in detail the block referenced C in FIG. 1 (i.e. a portion of the streamer referenced 20a).
The seismic method is based on analysis of reflected seismic waves. Thus, to collect geophysical data in a marine environment, one or more submerged seismic sources are activated in order to propagate omnidirectional seismic wave trains. The pressure wave generated by the seismic source passes through the column of water and insonifies the different layers of the sea bed. Part of the seismic waves (i.e. acoustic signals) reflected are then detected by the hydrophones distributed over the length of the seismic streamers. These acoustic signals are processed and retransmitted by telemetry from the seismic streamers to the operator station situated on the seismic vessel, where the processing of the raw data is carried out.
A well-known problem in this context is the localization of the seismic streamers. Indeed, it is important to precisely locate the streamers, in particular for:                monitoring the position of the hydrophones in order to obtain a satisfactory precision of the image of the sea bed in the exploration zone;        detecting the movements of the streamers with respect to one another (the streamers are often subjected to various external natural constrains of variable magnitude, such as the wind, waves, currents); and        monitoring the navigation of streamers, in particular in a situation of bypassing an obstacle (such as an oil barge).        
In practice, it is aimed to carry out an analysis of sea bed with a minimum number of passages of the vessel in the concerned area. For that purpose, the number of streamers implemented in the acoustic network is substantially raised. The aforesaid problem of localization of the streamers is thus particularly noticeably, especially in view of the length of the streamers, which may vary between 6 and 15 kilometers, for example.
Control of the positions of streamers lies in the implementation of navigation control devices, commonly referred to as “birds” (white squares referenced 10 in FIG. 1). They are installed at intervals that are not necessarily regular (50, 150, 300, or 450 meters for example) along the seismic streamers. The function of those birds is to guide the streamers between themselves. In other words, the birds are used to control the depth as well as the lateral position of the streamers. For this purpose, and as illustrated in FIG. 2, each bird 10 comprises a body 11 equipped with motorized pivoting wings 12 (or more generally means of mechanical moving) making it possible to modify the position of the streamers laterally between them (this is referred to a horizontal driving) and drive the streamers in immersion (this is referred to a vertical driving).
To carry out the localization of the seismic streamers, allowing a precise horizontal driving of the streamers by the birds, acoustic nodes are distributed along the streamers. These acoustic nodes are represented by hatched squares, referenced 14, in FIGS. 1 and 2. As shown in FIG. 1, some acoustic nodes 14 of the network are associated with a bird 10 (case of FIG. 2), and other are not.
The acoustic nodes 14 use underwater acoustic communication means (hereafter referred to as electro-acoustic transducers), allowing to estimate the distances between acoustic nodes (named here below “inter-node distances”). More specifically, these transducers are transmitters and receivers of acoustic signals, which can be used to estimate an inter-node distance separating two acoustic nodes (acting as sender node and receiver node respectively) arranged on two different streamers (which may be adjacent or not) as a function of an acoustic signal propagation duration measured between these two nodes (i.e. a travel time of the acoustic signal from the sender node to the receiver node). From the acoustic network, this thereby forms a mesh of inter-node distances allowing knowing accurate horizontal positioning of all the streamers.
Usually, each acoustic node comprises an electro-acoustic transducer enabling it to behave alternately as a sender node and a receiver node for the transmission and the reception, respectively, of acoustic signals. In an alternative embodiment, a first set of nodes act only as sender nodes and a second set of nodes act only as receiver nodes. A third set of nodes (each acting alternately as a sender node and a receiver node) can also be used in combination with the first and second sets of nodes.
The inter-node distance dAB between two nodes A and B can be typically estimated on the basis of the following equation: dAB=c·TAB, with:                node A acting as a sender node which transmits an acoustic signal S to node B acting as a receiver node (see example in FIG. 1, with acoustic signal S shown as an arrow between nodes referenced A and B);        TAB, the propagation duration elapsed between the emission instant and reception instant of the acoustic signal transmitted from the sender node A to the receiver node B (assuming that the receiver node and the sender node are synchronized); and        c, a “measured” or “estimated” value of sound speed (also referred to as underwater acoustic sound velocity) of the acoustic signal.        
Computation of an inter-node distance can be carried out, either by the navigation system (for positioning the set of hydrophones), or the node manager system (for providing useful information to the birds for horizontal driving), or the acoustic nodes themselves (in case they are equipped with electronics intended for this computation). The acoustic nodes are further synchronized by the node manager system through a wire communication bus placed within the streamers.
In particularly, the positioning of seismic streamers in deployment or retrieve phase (transitory phase in which one or several streamers is or are deploying in water or retrieving from water) is a recurring problem that engineers must face, notably to prevent from the risk of tangles of streamers.
Usually, to estimate the position of a seismic streamer, the seismic streamers are equipped with magnetic compasses and/or GPS (for “Global Positioning System”) receivers:                GPS receivers are installed at a few particular points such as on the towing vessel, the head and tail buoys connected to the streamers;        magnetic compasses deployed in greater numbers along the streamers in order to determine the deformations of the streamers between particular points.        
This known solution however requires a time-consuming process and is not implementable to estimate the relative position of a seismic streamer in deployment or in retrieve phase. It requires the total deployment of network of streamers to estimative the position of the seismic streamer with respect to one another.
Another known solution, based on an underwater acoustic measurement method, is described in patent FR 2 947 390. It consists in estimating an inter-node distance separating two acoustic nodes (arranged on two different streamers), acting as sender node and receiver node, respectively, according to a predefined acoustic sending and receiving sequence. However, when a seismic streamer is in deployment or retrieve phase, this solution requires that the user reconfigures the acoustic sequence periodically (for example at each launch of an acoustic node) in order to update the mesh of inter-node distances. The implementation of such a solution is therefore irksome and time-consuming.
It would therefore seem to be particularly worthwhile to estimate the position of a seismic streamer in deployment or retrieve with respect to another one or a group of another seismic streamers already deployed, without necessarily having recourse to dedicated devices or a periodic reconfiguration of the acoustic sequence.